The present invention relates to a method for forming novel carbon foams which faithfully reproduce the geometry of precursor unreticulated polyurethane foams. More particularly a method for forming strong, crack-free, vitreous carbon foams is described.
The prior art has described carbon structures prepared from carbon particles mixed with liquid thermosetting furan resins which are shaped under pressure in a mold and then carbonized as the unsupported shape outside of the mold. Illustrative are the carbon structures shown in U.S. Pat. No. 3,089,195 which describes carbonization by slow temperature increases of about 100.degree. F. (55.degree. C.) per hour up to 1000.degree. F. (538.degree. C.) to prevent cracking.
A method described for producing carbon structures (bulk density 0.02 to 0.8 gm/cc) in the prior art involves the foaming of polyurethane resins in admixture with liquid furan resin precursors and subsequent unsupported carbonization of the combination. Carbon or other elements or compounds may also be incorporated. Illustrative are U.S. Pat. Nos. 3,345,440; 3,574,548 and 3,635,676 wherein the carbonization is achieved in a minimum of 36 hours. As stated in U.S. Pat. No. 3,574,548, slow heating was used to avoid spontaneous cracking.
Another prior art method for producing carbon structures is described in U.S. Pat. No. 3,446,593 wherein it is stated that a viscous liquid furan resin is impregnated into a polyurethane foam having a density of 0.25 grams per cubic centimeter, and then some of the viscous liquid resin may be drained out. The resulting structure is left to rest, so that the furan resin is hardened, for 4 to 8 days. The cured foam-furan resin structure is described as having a weight which is 10 times that of the foam alone. The product is then carbonized in the unsupported form at an average heating speed of 5.degree. C. to 10.degree. C. per hour up to 1200.degree. C. Thus the combined procedure takes about 9 or more days including about 5 days of closely regulated heating. The reason for the long, closely regulated heating time is to attempt to prevent the structures from spontaneously cracking during carbonization. The carbon structures are decribed as having a density of the order of 0.1 and 0.4 gm/cc. The carbon structure described does not faithfully reproduce the geometry of the polyurethane foam because of the excess amount of undrained cured furan resin trapped in the foam cells and remaining on the foam surfaces.
In our applications Ser. Nos. 336,636 and 489,409, a process is described for producing reticulated carbon structures from reticulated flexible polyurethane structures infused with a furan resin or catalyzed furfuryl alcohol. The reticulated polyurethane structures are derived from polyurethane foams by the removal of the cell membranes which divide contiguous cells. In these patent applications, it was shown that excess furan resin on the surfaces of the strands caused cracking upon rapid heating to produce carbonization. The unreticulated polyurethane foams were not used in applicants' process because the cell membranes prevented the removal of the excess viscous furan resin throughout the foam volume, especially in the interior portions of the foam, when attempts were made to make commercially practical sizes of vitreous carbon blocks. The membranes act as barriers between the individual cells of the foam. Since unreticulated polyurethane foams are less expensive than reticulated polyurethane structures, a method was sought to produce carbon foams from them with good reproducibility and which avoids the limitations imposed by viscous resins.
It is therefore an object of the present invention to produce crack-free vitreous carbon foams by a method which requires only a few hours to produce them. Further it is an object of the present invention to provide a method which is simple and economic, particularly because of the elimination of the need for precise temperature change regulation and because of the rapid carbonization times. Further, it is an object of the present invention to produce strong, crack-free, carbon foams which faithfully reproduce the form of a polyurethane foam parent material. These and other objects will become increasingly apparent by reference to the following description and the drawing.